(1) Field of the Invention
This invention relates generally to the field of electric motors, and more particularly to an improved rotor for either a two-pole synchronous reluctance motor or a two-pole switched reluctance motor.
(2) Description of the Prior Art
Prior art synchronous reluctance motors and switched reluctance motors are known. It is also known in the art that the torque developed by an AC synchronous reluctance motor depends upon or is directly related to the ratio of the direct axis reactance to the quadrature axis reactance. The AC synchronous reluctance motor disclosed in U.S. Pat. No. 4,110,646 to Rao, entitled "AC synchronous motor having an axially laminated rotor" provides increased pull-in and pull-out torque by providing a rotor having a plurality of segments wherein each rotor segment is comprised of a plurality of magnetic sub-segments, each comprised of a plurality of laminations of soft magnetic material extending axially parallel to the axis of the shaft and non-magnetic material extending axially parallel to the axis of the shaft and positioned intermediate the laminations of soft magnetic material. The motor disclosed therein has a four pole rotor, but the teachings of Rao are applicable for rotors having any even-numbered pole rotors. Rao discloses that the direct axis reactance can be further increased by using grain-oriented steel as the magnetic material, but that this advantage can only be obtained in a rotor construction having axial laminations rather than radial laminations.
More specifically, as shown in FIG. 1, the prior art rotor 23 comprises a shaft 20 having a core 27 on which are mounted laminations 26 in separate stacks 21 (one for each flux path between the poles) around core 27. The separate axial laminations 26 are held in place by fasteners 22 and plates 24. The shape of the individual laminations 26 is best illustrated in FIG. 2. These sheets are curved so that the high permeability direction (i.e., the d-axis) of the grain oriented material from which the laminations 26 are made is parallel with a section of the rotor taken perpendicularly to the axis of rotation of the shaft 20, and the low permeability direction of the grain oriented material is parallel to the axis of the rotation of the shaft (i.e., the z-axis). Because of the necessary curvature of the laminations and the complicated mounting, motors with this type of lamination, constructed to take advantage of grain-oriented material, may be expensive or impractical to build for some applications. Moreover, axially laminated motors have required additional electronics to start, and a starting cage is difficult to provide for axially laminated motors.
Patents to El-Antably, specifically U.S. Pat. No. 4,459,502, entitled "Self-cascaded reluctance motor with axially laminated rotor," and U.S. Pat. No. 5,296,773, entitled "Composite rotor for a synchronous reluctance motor," also describe rotors in which grain-oriented material may be used. Both of the El-Antably patents also disclose axial rather than radial laminations, however.
U.S. Pat. No. 3,679,924 of Menzies describes a "radially stacked group of closely-nested thin flat sheets or strips . . . the sheets being preferably a grain-oriented steel having high unidirectional permeability." However, as shown, for example, in FIG. 3 of Menzies, the stacks comprise radially stacked, axial laminations rather than axially stacked, radial laminations. Thus, each of the pair 3 of stacks on opposite sides of the rotor in FIG. 3 of Menzies requires a retaining bar extending parallel with the rotor axis to hold the stack in a compressed condition with the aid of fastening bolts. Because of the complicated mounting and the fact that individual laminations are of varying sizes, a motor made with a rotor as described in Menzies may, in some applications, be too expensive or impractical to manufacture.
The inventions disclosed in each of the aforementioned patents are all, to at least some degree, relatively difficult, expensive, and/or impractical to manufacture, compared to the easier, less expensive, and very practical methods of punching and stacking conventional radial laminations. Ccmmercial punching dies exist that can easily and quickly manufacture such rotors.
It would therefore be desirable to provide a laminated core for a synchronous reluctance or switched reluctance motor that provides increased pull-in and pull-out torque by employing grain-oriented magnetic material, but which can be conveniently manufactured with radial laminations.
It is also known that the anisotropy of grain-oriented magnetic material is reduced when the material is subjected to low flux densities.
It would therefore be advantageous to provide a grain-oriented laminated rotor for a synchronous reluctance motor, wherein the rotor additionally accommodates a full squirrel-cage winding on the rotor to allow it to be line-started.